High-class woods such as Teak, Oak, Hickory, Walnut, Hinoki, Port-orford-cedar, and Sugi as well as middle-class woods have been used as materials to build houses etc. and to manufacture products for the interior, for example, chairs, tables and bookshelves. Especially in Japan, straight grain boards have been preferred to wave grain boards, and trees with knots are disliked. Many trees became useless the moment the trees were cut down. As shown in FIG. 12, in actual lumbering processes, a board 71 was cut out first crossing the central part of a lumber log 70 while a large knot 67, a middle knot 68, and a small knot 69 were avoided. Subsequently, the rest was cut radially and stepwise into boards 72, and a scrap part 73 was discarded. If a bar-shaped knot (conical knot) was found inside a lumbered board, the lumber was either thrown away or could be only used as lower-class (second-class) wood.
On the other hand, the usual method employed in Europe and the U.S. for making lumber boards from a log is to cut out in a tangential direction (parallel) to the annual ring from a cross-sectional viewpoint of the log. This method has a higher product yield in comparison with the Japanese method.
Other methods of manufacturing straight grain lumber are disclosed in Laid-open Japanese patent application No. (Tokkai Sho) 61-249702 and Laid-open Japanese patent application No. (Tokkai Hei) 6-91612 in which only straight grain lumber is chosen and laminated with the use of an adhesive.
However, the above-mentioned conventional method which has been employed in Japan had the problem that the product yield for obtaining lumber without knots was extremely low. Furthermore, there was also a problem that wooden products easily made partial cracks or shakes since they are made of one board.
On the other hand, although people in Europe and the U.S. tend to be not so particular about straight grain lumber in general like Japan, they prefer lumber without knots. As already mentioned, a method employed in Europe and the U.S. is to cut out in a tangential direction (parallel) to the annual ring from the cross-sectional view of the log. Nevertheless, since knot parts were disliked, they were either dumped or used only as second-class wood. As a result, it was also a problem in Europe and the U.S. that product yield for obtaining lumber without knots was low. In addition, there was also a problem that wooden products easily made partial cracks or shakes since they are made of one board.
In the methods disclosed in the above-mentioned Laid-open Japanese patent application No. (Tokkai Sho) 61-249702 and Laid-open Japanese patent application No. (Tokkai Hei) 6-91612, product yield from a log remains similarly low since only parts without knots are used.